The prolific preservation of small, aquatic animals called graptolites throughout Scotland has helped scientists date British rock strata. Graptolites were free-floating colonial animals. They commonly lived in deep water, and fed on plankton by filter-feeding. Graptolites were extremely adaptable creatures, evolving into distinct species at particular times, meaning that their fossils can be used to date surrounding rocks fairly precisely. Not only this, but they can also be used to estimate both water depth and temperature at particular localities over time.
Graptolite fossils are most commonly found as impressions left in Southern Upland shales in Scotland, and date back to the Silurian and Ordovician (485 – 419 million years ago). Clusters of ‘saw-blade’ fingers are often all that remains of the graptolite colonies flattened into the rocks. A gorge at Dob’s Linn, between Selkirk and Moffat, has become particularly noted for its excellent graptolite specimens.
Andrew McMillan, formerly field geologist with the British Geological Survey, writes:
Graptolites – the unsung sailors of ancient oceans
Graptolites are a group of extinct zooplankton animals lived in the earth’s oceans from 540 million years ago to 320 million years ago. In countless billions they dominated the upper layers of the ocean in tropical regions. Their slender skeletons, from a few millimetres to over 1 metre in length, are preserved as fossils in a variety of sedimentary rocks which are found on every continent except Antarctica. Many geologists over their careers have never found a graptolite yet the fossilized remains of these enigmatic creatures are abundant if you know where to look and familiarise yourself with the likely rock strata. Although the fossils are visible with the naked eye, you should remember to take a hand lens! Typically in a black mudstone they look like silvery grey pencil markings. Their appearance gave rise to the name graptolith from the Greek graphein = ‘to write’ and lithos = ‘stone’.
The excitement of finding a graptolite is palpable. Even seasoned palaeontologists have been known become ecstatic on making a new discovery! So engrossed in their fieldwork, I have known them to lose track of time and get lost while returning home in the dark. Why should this be so? Well, graptolites are not only locally abundant in certain rock types (sometimes difficult to find) but they provide the key to unravelling the stratigraphy or order in which the rocks were laid down. Because many species were short-lived (lasting only a million years or so) they can be used as zone fossils. And this is where the geology on our doorstep in the South of Scotland has had a significant part to play. Here graptolites provide a crucial part of the story of formation of the oceanic rocks (mainly greywackes and mudstones) of the Southern Uplands of Scotland. It was thanks to Charles Lapworth (1842-1920), a teacher in Galashiels, that the importance of graptolites was recognised. Using their unique species shapes he worked out the stratigraphy of the ‘typical section’ of mudstones at Dob’s Linn east of Moffat. His ‘Moffat Series’ of graptolite zones (first published in 1878) has been used, with refinement, to solve stratigraphical problems in Ordovician and Silurian rocks not only in Scotland but also throughout the world. The graptolite-bearing rocks at Moffat and elsewhere in the South of Scotland also contain many thin layers of bentonite, a clay mineral derived from repeated settlement of volcanic ash from eruptions around the world. Zircon crystals from these bentonites can be radiometrically dated adding a precision to the stratigraphy.
If you want to learn more about these fascinating fossils, look for a copy of Fossils Illustrated: Graptolites by Douglas Palmer and Barrie Rickards (Woolbridge: The Boydell Press, 1991); and for good places to visit in the Southern Uplands try Geology in south-west Scotland – an Excursion Guide, Editor Phil Stone (Keyworth: British Geological Survey with the Edinburgh Geological Society, 1996) and British Regional Geology: South of Scotland by P Stone, A A McMillan, J D Floyd, R P Barnes & E R Phillips (Fourth edition). (Keyworth, Nottingham: British Geological Survey, 2012)